Electric ignition system



Nov. 17, 1970 H. J. TYLER 3,540,817

ELECTRIC IGNITION SYSTEM L Filed Feb. 28, 1969 FUEL l8 BURNER 2 LOOOOOOO SATURABI-E LAW l4 CONTROL 5/ TRANSFORMER ""W e fi n 24 t 227ELECTROMAGNETIC FUEL VALVE FUEL INPUT INVENTOR HUGH .1. TYLER BYATTORNEYS United States Patent F 3,540,817 ELECTRIC IGNITION SYSTEM HughJ. Tyler, Santa Ana, Calif., assignor to Robertshaw Controls Company,Richmond, Va., a corporation of Delaware Filed Feb. 28, 1969, Ser. No.803,298 Int. Cl. F23n 5/00 11.5. Cl. 431-66 6 Claims ABSTRACT OF THEDISCLOSURE An electric ignition system comprising an electric fuelignitor coupled in series with the primary winding of a saturablecontrol transformer across an electric power supply. The secondarywinding of the transformer is coupled in series with a thermistor typeheat sensor, a control switch, and an electromagnetic fuel valve. Thecontrol transformer is designed to saturate with only a few voltsapplied to the primary winding, whereby nearly full line power isapplied to the electric ignitor and a constant control voltage issupplied to the transformer secondary circuit. The constant controlvoltage induced in the secondary winding energizes the fuel valve whenthe control switch is closed and the heat sensor resistance is lowindicating sufficient ignitor temperature.

BACKGROUND OF THE INVENTION The present invention relates to electricignition systems and, more particularly, to a fail-safe electricignition system wherein a saturable control transformer is utilized toprovide effective fuel flow control.

Fuel burner systems under automatic control require an ignition systemwhich will ignite the fuel when heat is called for and also prevent theflow of fuel to the burner when combustion does not occur. Conventionaldevices presently in use are controlled by thermoelectric elements orbimetallic devices which are relatively slowacting. Thischaracteristically long reaction time allows raw fuel to escape into thearea surrounding the burner where it mixes with air to create adangerously explosive condition.

To compensate for the above mentioned hazard, fast acting circuits havebeen developed to control the flow of fuel to the system. While suchcircuits have reduced the danger of raw fuel leakage, they too haveproven to be unsatisfactory under certain conditions of operation. Forexample, designers have had to resort to complex voltage regulatingcircuits in order to cope with adverse control circuit conditionscreated by fluctuating currents in the line used to supply electricalpower to the system. In addition, control circuits presently employed inconjunction with electric ignitor elements load down the ignitor whichcauses increased heat-up time and an increased overall system currentdrain. Attempted solutions to this problem have rendered the systemcircuitry even more complex and have limited the development ofpractical I and economical systems for many important applications.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to construct an electric ignition system having all theadvantages of similarly employed prior art systems while improvinginitial ignitor heat-up time and system control accuracy.

Another object of the invention is to provide an electric ignitionsystem having rapid fuel flow shut-off upon fuel burner flame outage.

This invention has the further object in the provision of an electricignition system which prevents the flow of fuel to a burner when theignitor temperature is below fuel igniting temperatures.

An advantage of this invention is the provision of a simple, yetreliable, electric ignition system having improved fail-safecharacteristics.

The present invention is summarized in that an electric ignition systemfor a fuel burner includes an electrical power source coupled toenergize an electric ignitor which is located adjacent the burner, and asaturable control transformer. The transformer includes a primarywinding which is coupled in series with the ignitor and a secondarywinding which provides an induced control voltage, and the transformeris operated in its saturated condition by the power source, whereby theprimary winding has negligible impedance and the control voltage ismaintained constant. Control means are coupled to the secondary windingof the transformer and located adjacent to the burner for sensing theignitor temperature and for enabling a flow of fuel to the burner whensaid ignitor is at fuel ignition temperatures.

These and other objects and advantages of the present invention willbecome more fully apparent from the following detailed description ofthe preferred embodiment of the invention when considered in conjunctionwith the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The figure illustrates a schematiccircuit diagram of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing,there is shown an electric ignitor element 10 which is located adjacentto a fuel burner, shown schematically as 12, and is coupled in serieswith the primary winding of a saturable control transformer 14. Theseries combination of ignitor 10 and transformer 14 is further coupledto a source of electrical power, represented by power lines 16, throughan overload fuse 18 and a main power switch 20. The series circuit thusfar described provides energization for the ignitor element and thecontrol circuitry to be described below, and will be referred to hereinas the primary circuit of the system. 7

The secondary winding of saturable control transformer 14 provides acontrol voltage for regulating the flow of fuel to the burner 12 iscoupled in series with the energization winding of an electromagneticfuel valve 22, a negative temperature coetficient thermistor 24, and a.control switch 26. Electromagnetic fuel valve 22, when energized,establishes a flow of fuel from a fuel input source (not shown) throughthe fuel valve to fuel burner 12 via fuel conduit 28.

Although many varied applications of the circuitry disclosed herein arecontemplated, the operation of this invention can best be described byconsidering a typical installation thereof in a heating system.

When operation of a heating system employing the invention is desired,main power switch 20 will be closed thereby permitting current to flowfrom power source 16 to the primary circuit, comprising ignitor 10, fuse18, and the primary Winding of transformer 14. Transformer 14 is asaturable control transformer and is designed to operate in a saturatedcondition upon the application of only a few volts to the primarywinding thereof.

It is important to note at this point that the impedance of the primarywinding of saturable control transformers of the type employed by thepresent invention varies in an inversely proportional relationship withthe saturation level thereof. Thus, when such transformers are operatedin a fully saturated state, the impedance of the primary winding becomesquite small and consequently reduces the overall current drain of thesystem as well. as the potential drop across the primary winding.

Since transformer 14 is always operated in a fully saturated condition,the impedance of the primary winding thereof will be small therebypermitting the application of substantially full line voltage to theelectric ignitor. Since the ignitor element of the present invention istherefore operated at full voltage, it will reach fuel ignitiontemperatures more rapidly than heretofore accomplished. This feature ofthe invention assures initial ignition, provides rapid reignition aftera momentary electrical failure in the primary circuit, and thusincreases the inherent fail-safe characteristics of the overall system.

Continuing with the description of the operation of the invention, afterthe system has been turned on by main power switch 20 and energizationof the primary circuit has been initiated, a control voltage will beinduced in the secondary winding of transformer 14 to energize theelectromagnetic fuel valve 22 and establish the flow of fuel to thesystem burner. Before the fuel valve can be actuated, however, twoconditions must be satisfied. The first is that control switch 26 beclosed as to complete the circuit connection between the fuel valve andthe secondary winding of the transformer. Switch 16 may be any ofseveral devices depending upon individual installation requirements andis shown as a single-pole singlethrow switch solely for the sake ofclarity. In a conventional heating system, for example, switch 16 wouldbe responsive to a thermostat (not shown) which monitors the temperatureof the area to be heated and automatically actuates the switch when ademand for heat exists.

The second condition is that the resistance of thermistor 24 besufliciently low to allow enough current to flow through theenergization winding of the electromagnetic fuel valve for actuationthereof. Thermistor 24 is located adjacent to the ignitor element sothat its temperature closely follows the ignitor temperature, and isdesigned to allow sufficient current to flow to the fuel valve onlyafter the ignitor has reached fuel ignition temperatures.

Therefore, the burner will receive fuel for combustion only when (a) ademand for heat exists and (b) the ignitor element is at fuel ignitiontemperatures. In addition, since the transformer is maintained in asaturated condition at all times during operation, the control voltagepro: vided by the secondary winding of saturable transformer 14 remainsconstant and is independent of any fluctuations in line voltage frompower source 16. Since the volt-. age across fuel valve 22 determineswhether it will "be open or closed, it is extremely important to preventline fluctuations from influencing the control circuit voltage. Thus, byutilizing a saturable control transformer as shown, the effect of powerline voltage changes is minimized so that the only variations in thecontrol circuit voltage of the present invention will be those producedby the thermistor 24 in response to the temperature of ignitor 10, asdesired. This feature allows more accurate design of the control circuitand increases the safety of the system. In describing the fail-safecharacteristics of the invention, it can be seen from the figure that afailure in any component causing an open circuit will immediately removeactuating potential from the fuel valve to rapidly stop the flow of fuelto the burner. In addition, a short circuit produced by a failure in thefuel valve 22, switch 26 or transformer 14 will also remove theactuating potential from the valve. Finally, a short circuit in theignitor 10 will cause an overload condition in the primary circuit, dueto the low impedence of the primary winding of transformer 14 whensaturated, which will cause fuse 18 to rapidly break the energizingcurrent path to the ignition system from power source 16.

Thus, there is provided an electric ignition system which is simple, andyet includes additional fail-safe features not found in present systems.The invention therefore makes possible the design of many usefulappliances requiring only minimal maintenance and at the same timeproviding increased consumer safety.

Inasmuch as the present invention is subject to many variations,modifications and changes in detail, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

What is claimed is: 1. An electric ignition system for a fuel burner,comprising:

an electrical power source; an electric ignitor coupled to said powersource for energization thereby and located adjacent said burner; I

a saturable control transformer coupled to said power source andincluding a primary winding coupled in series with said electricignitor, and a secondary winding for providing an induced controlvoltage, said transformer being operated in its saturated condition bysaid power source whereby said primary winding has negligible impedanceand said control voltage is maintained constant; and

control means coupled to the secondary winding of said transformer andlocated adjacent said burner for sensing the temperature of saidelectric ignitor and for enabling a flow of fuel to said burner whensaid ignitor is at fuel ignition temperatures.

2. The invention as recited in claim 1, wherein said control meansincludes a thermistor having a negative temperature coefficient ofresistance.

3. The invention as recited in claim 2, whereinsaid control meansfurther includes an electromagnetic fuel valve electrically coupled inseries with said thermistor to form a series network, said seriesnetwork being coupled in parallel with the secondary winding of saidtransformer.

4. The invention as recited in claim 3, wherein said control meansfurther includes a thermostatically actuated electrical switch coupledin series with said fuel valve and said thermistor.

5. A rapid ignite fail-safe electric ignition system, comprising:

a fail-safe saturable control transformer having a saturationleveldependent upon applied voltage, including a primary Winding having animpedance which is inversely proportional to the saturation level of thetransformer, and a secondary winding producing an induced controlvoltage which is constant when the transformer is fully saturated;

an electric ignitor coupled in series with the primary winding of saidtransformer;

an electromagnetic fuel valve;

heat sensing means coupled to said fuel valve and the secondary windingof said saturable transformer to form a closed-loop series circuit, forsensing the temperature of said ignitor and for enabling said fuel valvewhen said ignitor is at fuel igniting temperatures; and 1 electricalpower supply means coupled to said ignitor and said transformer forenergizing said system and for maintaining said transformer in its fullysaturated state.

6. The invention as recited in claim 5, wherein said heat sensing meanscomprises a thermistor having a nega tive temperature coefiicient ofresistance.

References Cited UNITED STATES PATENTS 3,454,345 7/1969 Dyre 43l-663,480,374 11/1969 Lamb et al. 43166 EDWARD G. FAVORS, Primary ExaminerUS. Cl. X.R. 31798; 431-254

